The pseudo hot-melt prepregging process is a desirable way of producing reinforced polyimide laminates and shapes. In this process a very viscous hot solution of a curable polymeric precursor is spread onto rolls of release paper. After cooling to room temperature the coating becomes a flexible resinous solid. It is subsequently transferred to a continuous fibrous substrate by passing the coated web and the fibrous substrate through a bank of hot rolls to impregnate the coating onto the substrate. The product (often referred to as prepreg tape) is sold according to grade or areal weight of fiber which usually ranges from 50 to 300 grams per square meter and more typically 100 to 200 grams per square meter. The trend in such products is use of amounts of curable resin in the range of 25 to 50%, typically 30 to 35%. Tolerances in the product are critical and typically fall in the range of plus or minus 3 percent.
Polyimides made from 2,2-bis[(4-aminophenoxy)phenyl]hexafluoropropane ("4-BDAF") and their high temperature properties are well known. See for example U.S. Pat. Nos. 4,111,906, 4,203,922 and 4,477,648 and the paper by Peter Delvigs entitled "371.degree. C. Mechanical Properties of Graphite/Polyimide Composites" prepared for the Second International Conference on Polyimides sponsored by the Society of Plastics Engineers, Inc., Ellenville, N.Y., Oct. 30-Nov. 1, 1985 (NASA Technical Memorandum 87122).
Difficulties are encountered when an attempt is made to form polyimide precursor solutions of 4-BDAF and tetracarboxylic acid derivatives (e.g., diesters of pyromellitic acid or 3,3',4,4'-benzophenonetetracarboxylic acid) for use in the pseudo hot-melt prepregging process. Although the solubility of these polyimide monomers in certain dipolar aprotic solvents at room temperature is quite high, even solutions in these solvents have too low a viscosity to be useful in the process. While the solubility of these monomers in the solvents can be increased to 60% or more by warming the solutions, the solutions deposit solids on cooling to room temperature and are consequently unsuited for prepregging using the pseudo hot-melt process.